How, then, are we to account for the fact that Darwin attributed to natural selection the power to cause divergence of character? The answer is sufficiently simple. _He does so by tacitly invoking the aid of some other form of homogamy in every case._ If we carefully read pp. 86-97 of the _Origin of Species_, where this subject is under consideration, we shall find that in every one of the arguments and illustrations which are adduced to prove the power of natural selection to effect "divergence of character," he either pre-supposes or actually names some other form of homogamy as the originating cause of the diversity that is afterwards presented to natural selection for further intensification. To give only one example. At the starting-point of the whole discussion the priority of such other forms of homogamy is assumed in the following words:--
But how, it may be asked, can any analogous principle [to that of diversity caused by artificial selection] apply in nature? I believe it can and does apply most efficiently (though it was a long time before I saw how), from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.
Now, without question, so soon as segregate breeding in two or more lines of homogamy has been in any sufficient degree determined by some "change of structure, constitution, or habits," natural selection will forthwith proceed to increase the divergence in as many different lines as there are thus yielded discriminately isolated sections of the species. And this fact it must have been that Darwin really had before his mind when he argued that diversification of character is caused by natural selection, through the benefit gained by the diversified forms being thus "enabled to increase in number." Nevertheless he does not expressly state the essential point, that although diversification of character, _when once begun_, is thus _promoted_ by natural selection, which forthwith proceeds to cultivate each of the resulting branches, yet diversification of character can never be _originated_ by natural selection. The change of "structure," of "constitution," of "habits," of "station," of geographical area, of reciprocal fertility, and so on--this change, _whatever_ it may have been, must clearly have been antecedent to any operation of natural selection through the benefit which arose from the change. Therefore the change must in all cases have been due, in the first instance, to some other form of isolation than the superadded form which afterwards arose from superior fitness in the possession of superior benefit--although, so long as the prior form of isolation endured, or continued to furnish the necessary condition to the co-operation of survival of the fittest, survival of the fittest would have continued to increase the divergence of character in as many ramifying lines as there were thus given to its action separate cases of isolation by other means.